使用 ARC 并针对 iOS 4.0 时如何替换弱引用？
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How do I replace weak references when using ARC and targeting iOS 4.0?
提问by Mason G. Zhwiti
I've begun developing my first iOS app with Xcode 4.2, and was targeting iOS 5.0 with a "utility application" template (the one that comes with a FlipsideViewController).
我已经开始使用 Xcode 4.2 开发我的第一个 iOS 应用程序，并使用“实用程序应用程序”模板（FlipsideViewController 附带的模板）针对 iOS 5.0。
I read that since ARC is a compile-time feature, it should be compatible with iOS 4 as well, so I attempted to target my app to 4.3, and try compiling it. When I do so, I get this error:
我读到，因为 ARC 是一个编译时功能，它也应该与 iOS 4 兼容，所以我尝试将我的应用程序定位到 4.3，并尝试编译它。当我这样做时，我收到此错误：
FlipsideViewController.m: error: Automatic Reference Counting Issue: The current deployment target does not support automated __weak references
FlipsideViewController.m：错误：自动引用计数问题：当前部署目标不支持自动 __weak 引用
It is referencing this line:
它引用了这一行：
@synthesize delegate = _delegate;
That variable is declared as:
该变量声明为：
@property (weak, nonatomic) IBOutlet id <FlipsideViewControllerDelegate> delegate;
I understand that "weak references" are not supported in iOS 4, but I don't really understand why I would want to use a weak reference to begin with, nor can I figure out how I would rewrite things to avoid using it, while still taking advantage of ARC (after all, it's supposed to work with iOS 4 AND 5 right?)
我知道 iOS 4 不支持“弱引用”，但我真的不明白为什么我要使用弱引用开始，也无法弄清楚如何重写内容以避免使用它，而仍在利用 ARC（毕竟，它应该适用于 iOS 4 和 5，对吗？）
回答by Brad Larson
To target the older OS, you can use unsafe_unretainedinstead of weakin your property declaration, and it should mostly work the same way. weakreferences nil themselves when their target goes away, but unsafe_unretainedleaves open the possibility that the object you're linking to could turn into a dangling pointer when it is deallocated. The latter is the same behavior as if you had used assignas a property declaration in manual memory management.
要针对较旧的操作系统，您可以在属性声明中使用unsafe_unretained而不是weak，它应该以相同的方式工作。 weak当他们的目标消失时引用 nil 自己，但unsafe_unretained留下了你正在链接的对象在被释放时可能变成悬空指针的可能性。后者与您assign在手动内存管理中用作属性声明的行为相同。
You do this to avoid retain cycles, which I mention in my answer here. You don't want to have a strong pointer to something that might have a strong pointer back to the original object. Then nothing would get released properly.
您这样做是为了避免保留周期，我在此处的回答中提到了这一点。您不想拥有指向可能具有指向原始对象的强指针的强指针。那么什么都不会被正确释放。
回答by rpetrich
If only using weak references for additional safety, manually call the new runtime functions if they're available and fallback to simple assignment on __unsafe_unretainedvariables if not.
如果仅使用弱引用来提高安全性，请在可用时手动调用新的运行时函数，如果不可用则回退到对__unsafe_unretained变量进行简单赋值。
ZWRCompatibility.hwill simplify this somewhat.
ZWRCompatibility.h会稍微简化一下。
回答by nschum
Thanks to Mike Ash's compatibility library PLWeakCompatibilty, you can now simply use __weak on iOS 4.x, as well.
感谢 Mike Ash 的兼容性库PLWeakCompatibilty，您现在也可以在 iOS 4.x 上简单地使用 __weak 。
It's incredibly easy to configure and requires no additional consideration or effort over 5.x.
它非常容易配置并且不需要额外的考虑或超过 5.x 的努力。
